1. Field of the Invention
The invention relates to a reduction projection objective for imaging an effective object field arranged in an object surface of the projection objective onto an effective image field arranged in an image surface of the projection objective and a projection exposure apparatus including such projection objective.
2. Description of the Related Art
Reduction projection objectives are, for example, employed in projection exposure apparatuses used for fabricating semiconductor devices and other types of micro devices and serve to project patterns on photomasks or reticles, hereinafter referred to generically as “masks” or “reticles,” onto an object having a photosensitive coating with ultrahigh resolution on a reduced scale.
A projection exposure apparatus for microlithography includes an illumination system for illuminating the mask, arranged in an exit surface of the illumination system, and a projection objective, following the mask, which is designed for creating an image of the pattern of the mask in the image surface of the projection objective, where the object to exposed is arranged. The exposure radiation provided by the projection objective is incident on the photosensitive substrate within an effective image field at an image-side numerical aperture NA of the projection objective.
It is generally desired that the projection objective has a sufficient correction status of all imaging aberrations for all field points within the effective image field in order to obtain the desired performance. The aberrations include chromatic aberrations, image curvature aberration, distortion, spherical aberrations, astigmatism etc. The entire effective object field should be imaged into the image surface without vignetting, i.e. without loss of information regarding critical field points, typically from the edge of the field.
In the manufacture of highly integrated semiconductor devices it is often desired that at least some layers of a three dimensionally structured semiconductor device are produced at conditions where the image-side numerical aperture and the selected operating wavelength λ of ultraviolet radiation are sufficient to obtain a relatively high resolution R=k1 (λ/NA), where k1 is an empirical constant depending on certain process parameters. For example, resolutions R<100 nm may be required for critical layers.
Projection objectives optimized for high resolution typically employ operating wavelengths λ<260 nm, e.g. λ=248 nm or λ=193 nm. Since there are only few materials sufficiently transparent in that wave-length region (e.g. synthetic quartz glass (fused silica) and crystalline fluorides, such as calcium fluoride) and the Abbe numbers of those materials are close together, the correction for chromatic aberrations (color-correction) becomes difficult.
Further, as the available transparent material with sufficient optical quality are in tight supply, material conserving designs are desirable.
In optical lithography, high resolution and good correction status have to be obtained for a relatively large, virtually planar effective image field. Providing a flat image requires lens elements with opposing lens powers. That leads to stronger lens elements, more system length, larger system glass mass, and larger high-order image aberrations that result from the stronger lens curvatures. Effective means for flattening the image field, i.e. for correcting the Petzval sum in projection objectives for microlithography are therefore needed.
Concave mirrors have been used for some time to help solve problems of color correction and image flattening. A concave mirror has positive power, like a positive lens, but the opposite sign of Petzval curvature. Also, concave mirrors do not introduce color problems. Therefore, catadioptric systems that combine refracting and reflecting elements, particularly lenses and one or more concave mirrors, are primarily employed for configuring high-resolution projection objectives of the aforementioned type.
The resolution which can be obtained in an imaging process is only one of a large number of criteria to be observed in an projection exposure process. Often it is desired to improve productivity of a manufacturing processes involving the projection objective. Specifically, it may be desired for economical reasons to maximize the number of substrates which can be exposed in a given time interval, e.g. to maximize the throughput of the projection exposure process.
The article “Optical Lithography—Thirty years and three orders of magnitude” by J. H. Bruning, SPIE vol. 3049, pages 14 to 27, describes the evolution of optical lithography tools in view of conflicting requirements regarding the resolution to be obtained and the rapid increase of substrate size to be exposed in the field of manufacturing integrated circuits. The author describes that industry moved from contact printing to projection printing in the early 1970's as the complexity of integrated circuits increased and defects became critical. After a time where full wafers where printed at 1:1 magnification, the rapid increase in wafer size was accommodated by annular field scanning using 1:1 imaging mirror systems. Further decreased linewidths requirements and overlay budgets forced the introduction of reduction “step-and-repeat” printing of small blocks of chips in the late 1970's. Further demands for smaller linewidths and larger chips have driven optical lithography to shorter wavelength and to scanning the chip in a “step-and-scan” printing mode. In both processes reduction projection objective have been used, starting with reduction ratios of 10:1 in the mid-1970's and tending to nowadays reduction ratios of 5:1 or 4:1.
Projection objectives for a step-and-repeat process, often denoted as “stepper lens” or “stepper objective”, are typically designed for a quadratic or a low aspect ratio rectangular effective image field. A popular all-refractive (dioptric) projection objective for a step-and-repeat process was designed for a 22 mm×27.4 mm effective image field centered around the optical axis and NA=0.60. Under these conditions, the circular area on the image side for which the projection objective must be sufficiently corrected has a diameter of about 35.1 mm corresponding to a maximum image field height Y′ of about 17.6 mm.
Scanning systems for a reduction step-and-scan process have been developed for cases where the maximum size of the effective image field is not sufficient to cover the desired size of the area to be exposed on the substrate, e.g. the area of the chip on a wafer. Using a slit-shaped effective image field having a high aspect ratio between the widths (perpendicular to the scanning direction) and the height (in scanning direction) allows to expose larger areas on the substrate by successively exposing adjacent areas on the substrate while the mask and the substrate are moved relatively to each other in a scanning direction.
A popular all-refractive projection objective for a scanner-system was designed for a 26 mm×12 mm effective image field (effective image field diameter ca. 29 mm, maximum image field height ca. 14.5 mm) and an image-side numerical aperture NA=0.65. Due to the slit-shaped effective image field the maximum image field height Y′ for which the projection objective must be corrected is smaller than that needed for a low aspect ratio rectangular or a quadratic effective image field having the same width.